Unfortunately I don't think there is any, but the simple math of it is this: 1.5-2% of their body weight (so for a normal sized horse, between 15 and twenty pounds of hay/grass per day) in forage.
For hard feeds, it depends on the type of feed. There are feed charts on the back of every bag, and you will have to multiply the decimal (will say something like ".25-.50 pounds of feed for 100lb of body weight per day", which again with a 1000 pound horse is between 2.5 lbs and 5 lbs a day) by your individual horses body weight, according to how much work they do.
Tblvr is correct, you're not going to find any type of calculator to tell you how to feed your horse. It would be impossible for someone to generate one that would work on every single horse in the world. The only thing that is the same from horse to horse, is the fact that they are horses, and that is pretty much it. There are way too many factors that affect how a horse has to be fed and how much, outside of the horses weight, and activity level.
Anyone who has completed an equine nutrition program will have the grid to calculate feed needs and can recommend feeding options. I do offer this as part of my practice. If you would like a feeding calculation pm me and we can discuss.
Old thread or not, the calculator is pretty cool. I wouldn't use it 100% but as a general guide to get an idea of what you need before looking into feeds yourself I can see it being beneficial. So thank you for that, forest.